High-transparency coded apertures in planar nuclear medicine imaging: Experimental results

Coded apertures provide an alternative to collimators in gamma-ray imaging. Advances in the field of coded apertures have lessened the artifacts that are associated with the near-field geometry of nuclear medicine. Nevertheless, image resolution and the manufacturing techniques that are available are constrained by the thickness of the aperture material. Thickness artifacts result. Thin apertures are theoretically desirable, but high transparency results in a loss of contrast. Together with detector quantization effects, this leads to noise in the reconstructed image. Provided that the gamma camera has a sufficient bit-depth, and an appropriate number of counts of radioactivity are obtained, high transparency need not reduce the signal- to-noise ratio. An opaque coded aperture, with an attenuation of 97 %, was constructed by laser drilling a tungsten sheet. A highly transparent coded aperture, with an attenuation of 29 %, was obtained by laser ablating tungsten foil. A specialized aluminium gamma camera frame matches the coded aperture to the mounting mechanism of the gamma camera, and facilitates both alignment and rotation of the coded aperture - the latter for the application of a near-field artifact reduction technique. This paper includes simulation of the effect of count statistics on high- transparency coded aperture images, and presents experimental planar phantom-based results for both the opaque and the highly transparent coded apertures. The results are comparable to the simulations, and support the concept of high-transparency coded apertures in diagnostic nuclear medicine.